Batteries having gel polymer electrolytes or solid polymer electrolytes are known. See U.S. Pat. Nos. 5,418,091 & 5,460,904, and Linden, D., Handbook of Batteries, 2nd Edition, McGraw Hill, New York, N.Y. (1995), pgs. 36.37-36.42 et seq. It has been suggested that microporous membranes can be used in the assembly of polymer electrolyte batteries. See U.S. Pat. Nos. 5,518,838; 5,604,660; 5,631,103; 5,639,573; 5,681,357; 5,688,293; 5,750,284; 5,837,015; 5,853,916; 5,658,685; 5,849,433; 5,665,265; 5,716,421; and 5,834,135; Gozdz, A., “Plastic Li-Ion (PLiON™) Rechargeable Cells with Bonded Microporous Separator,” Telecordia Report, April 2000; and Gozdz, A., et al., “Fabrication and Performance Characteristics of Plastic Li-Ion Batteries With Bonded Untreated Microporous Polyolefin Separators,” 198th Meeting of the Electrochemical Society, Oct. 22-27, 2000. Separators, i.e., coated microporous membranes, designed specifically for use in such batteries are known. See U.S. patent application Ser. No. 09/016,024 filed Jan. 30, 1998, and WO 99/54953 claiming priority of Apr. 20, 1998.
WO 99/54953 discloses a composite electrolyte comprising a microporous membrane and a coating of an unplasticized, porous organic polymer containing a compound that has a dissociable lithium ion. The non-plasticized coating partially penetrates the pores of the membrane. The porous organic polymer may be polyvinylidene fluoride. In the examples, 15% of the pore volume was filled when the coating was polyethylene oxide (PEO), 35% of the pore volume was filled when the coating was polyvinylidene fluoride (PVDF); and 20% of the pore volume was filled when the coating was polytetrafluoroethylene (PTFE).
U.S. Pat. Nos. 5,518,838 and 5,604,660 disclose an electrolyte system comprising a solid polymer electrolyte impregnated into a porous separator.
U.S. Pat. No. 5,631,103 discloses an electrolyte system comprising a homogeneous mixture of an inert filler (for example, polymers or inorganic materials) and a gel-forming polymer, but no microporous membrane.
U.S. Pat. No. 5,639,573 discloses an electrolyte system comprising a microporous membrane and a gel-forming polymer, and the gel-forming polymer “extends at least partially into, and preferably through, the pores.” Also see FIG. 2. Also see U.S. Pat. Nos. 5,681,357; 5,688,293; 5,750,284; 5,837,015; and 5,853,916; and U.S. application Ser. No. 09/016,024 filed Jan. 30, 1998.
U.S. Pat. Nos. 5,658,685 and 5,849,433 disclose an electrolyte system comprising a polymeric blend of a gel-forming polymer and an inert polymer, but no microporous membrane.
U.S. Pat. No. 5,665,265 discloses an electrolyte system comprising a nonwoven and a gelling polymer expanded between the fibers of the nonwoven.
U.S. Pat. Nos. 5,716,421 and 5,843,153 disclose an electrolyte system comprising a microporous membrane and a gelling polymer “wherein the gelling polymer and the electrolyte seeps or is forced into the pores.”
There is a desire on the part of some battery manufacturers to move from liquid electrolytes to gel or solid electrolytes. One reason for this move is that cells made with gel or solid electrolytes may be moldable in to a variety of shapes. Another reason is to prevent the leakage of the electrolyte. This move, however, has been hindered by the fact that the conductivity of the gel or solid electrolyte is much less than that of the liquid electrolyte. To compensate for the lower conductivity of the gel and solid electrolytes, thinner electrolytes are required. Thinner electrolytes, however, are detrimental to the manufacture of the batteries because of their low mechanical strength. Accordingly, battery manufacturers have had to compromise. That compromise is the inclusion of a microporous membrane in the electrolyte. Inclusion of the microporous membrane has enabled the manufacture of these batteries. Gozdz, Ibid. At first, it was suggested that the gel-forming polymer should fill the pores of the membrane. See U.S. Pat. Nos. 5,639,573; 5,681,357; 5,688,293; 5,716,421; 5,750,284; 5,837,015; and 5,853,916. Later, it was suggested that the gel-forming polymer should partially fill the pores of the membrane. See PCT WO 99/54953.
Although, these separators perform well, there is still a need to continue to improve the conductivity of these coated separators for gel or polymer batteries.